docs: Update DesignToken documentation to address API changes from #6171

[nicholasrice]

🙋 Documentation Request

#6171 refactored DesignToken and changed the API. Documentation should cover:

• Changes to the subscribe & unsubscribe methods (no "target" argument anymore)
• DesignToken Resolver pattern
• How to set values for FASTElements (set values in connectedCallback / after connected to the DOM)
• Migration guide

💁 Possible Solution